The central auditory system diverges into multiple parallel ascending pathways in the brain stem. Most of these pathways terminate in the inferior colliculus (IC), but a few bypass it. Understanding the specializations within each pathway and the relationships among the pathways at their target, the IC, is a fundamental problem in understanding the structure of the central auditory system. The echolocating bat provides a special opportunity for studying this problem. In the lower brain stem of the bat the origins of some of these pathways are hypertrophied or highly differentiated so that their connections can be readily examined. These parts of the bat's auditory pathway may serve as models of their homologues in all mammals. The bat's production of an easily characterized sonar signal provides information not available in other mammals about the sound stimuli that are processed by the auditory system. The general goal of the proposed research is to answer the question of how different ascending auditory pathways from the lower brain stem converge or diverge at the inferior colliculus in echolocating bats, how these projections are related to the structural, tonotopic and cytochemical organization of the IC, and what role individual pathways may play in processing information about sound stimuli. An additional goal is to examine the physiology of pathways that bypass the IC and may play an important role in auditory-motor connections. Specific experiments are designed to compare physiological response properties and ascending connections from the different nuclei of the lateral lemniscus; these nuclei are hypertrophied and highly organized in echolocating bats. Other experiments are designed to compare patterns of connections with physiological response properties, distribution of endogenous substances and intrinsic connections in the IC. The method combine electrophysiology and neuronal tracing techniques using (3H)-leucine, phaseolus vulgaris leucoagglutinin, HRP and wheat germ agglutinin conjugated to HRP. The same micropipette is used to inject the tracer and to record neural responses, so that the injection site is characterized according to physiological measures. Information about the structure of the bat's auditory pathways provides information about possibilities and limitations of the system in processing auditory signals. The information is essential to the diagnosis of pathologies of hearing.